Birds squawk and chirp to attract mates and warn of danger. But much of their intelligent chatter has until now eluded human comprehension.

The black-capped chickadee not only warns its flock of danger but also communicates the predator's size and relative threat, a new study finds.

All with a familiar chick-a-dee-dee-dee -- plus a few more dees.

Imminent danger?

A cat on the ground might elicit five or 10 dees. But something closer and capable of an aerial attack could generate nearly two dozen closing notes.

"With something really dangerous, such as a pygmy-owl perched near some chickadees in our aviary, we heard as many as 23 added dees," said Chris Templeton, a biology doctoral student at the University of Washington and lead author of the study.

The acoustic signatures of the calls change too, in ways humans can't notice.

The results are detailed in the June 24 issue of the journal Science.

Your backyard

Black-capped chickadees are common in much of North America, and might be in your backyard right now, according to scientists at Cornell University. They are about 5 inches long and are very active. Look for a black cap and white cheeks.

Scientists had already described their call as one of the most complex in the animal kingdom. A chickadee can tell of individuals it spots or entire flocks it recognizes, previous studies showed.

The new research was done in an outdoor, semi-natural aviary with 15 live predators perched or on leashes.

Small but dangerous

Chickadees recognize a predator's threat status based on its size and agility, the study found. And, they know, bigger isn't always badder. Like Tweetie Bird taunting Sylvester the Cat, they can virtually ignore some not-so-dangerous predators.

"A pygmy-owl is more dangerous to a chickadee than a great horned owl that has a large hooked beak and big talons," Templeton explained. "A great horned owl going after a chickadee would be like a Hummer trying to outmaneuver and catch a Porsche."

The test chickadees paid no attention to a nearby and harmless bobwhite quail, suggesting the songbirds also recognize various species.